About Time

Time is a concept that has long perplexed philosophers and scientists. Time, space,
matter, and energy are all bound together to make the fabric of our universe. We are
embedded in time, and cannot think or speak without reference to time. The exact definition
of time may be elusive, but we are all interested in measuring and marking time. So
how do we tell time? How did our ancestors mark time? The science of timekeeping is
called horology.

Time has been measured, divided, and subdivided into many units - nanoseconds, microseconds,
milliseconds, seconds, minutes, hours, days, weeks, lunar months, solar months, seasons,
years, decades, centuries, millennia, stages, epochs, periods, eras, and eons. Time
has been marked by the crowing of roosters, sunrise, sunset, and midday. Many clever
inventions have been devised to keep time, including sandglasses, water clocks, and
graduated oil lamps. Then there were pendula clocks; and spring, battery, and electric
clocks or watches. Today, precise time is kept by a number of atomic clocks at governmental
agencies such as the U.S. Naval Observatory in Washington, D.C. Since 1967, an atomic
second has been defined as the interval of time it takes a cesium-133 atom to "vibrate"
(i.e., release radiation during electron transitions between energy levels) 9,192,631,770
times.

Atomic clocks were invented in an attempt to control for the variations in solar time.
But even with their precision and constancy, it is still necessary to periodically
add "leap seconds" to atomic clocks to compensate for the Earth's gradually slowing
rotation (spin) and lengthening of the day (due to tidal friction), so that our clocks
and calendars remain close together. On average, one leap second is added about every
year to year and a half.

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